I have one. Can't think why I haven't blogged this before!
I had a brief look at SL several years back, but the interface was clunky and I wasn't enthused enough to continue. A few months ago I tried again, found it much improved, and as soon as I got beyond the 'new player experience' I was hooked.
I'll assume you've at least read the media accounts of Second Life, which indicate that it is populated by pedophiles, virtual conferencing business suits and land sharks. Well those people are there in small numbers, but the bulk of 'residents' do other things.
The difference between SL and other online games is that players have the in world editing program. Almost everything you see in SL is constructed by players - buildings, avatars, vehicles and so on. There is a scripting language that enables them to add animations, sound and video. So what do they do with this? Well actually they do what people do in Real Life...
Have sex. Sex in SL is (for me at least) mostly fun, but now and then a good animation with attractive avatars and a good partner can be surprisingly erotic. For others, I gather it works quite well as a masturbation aid, and good luck to them! It's also interesting as part of the overall kick I get out of roleplaying games, which is to be 'someone else'.
Learn new skills. In SL this means finding out how to Create things, something I've only scratched the surface of so far. I've extensively rebuilt my castle, modified clothes, can do textures but my scripting is still Read Only.
Make money. Well, not so far. as in RL that requires a good product, marketing and after sales service with the added twist that the relative values of things are entirely different. A castle is cheaper than a sex bed because of the time spent creating them, since raw material cost is zero.